The present invention relates to a hydrogen storage material, comprising a magnesium-containing intermetallic compound capable of forming a hydride with hydrogen. The invention also relates to an electrochemical cell comprising such a hydrogen storage material.
Rechargeable batteries can be discharged and then restored to their original state for re-use. Rechargeable batteries are increasingly used in portable electronic equipment such as telephones, walkmans and computers. A known type of rechargeable battery is the so-called rechargeable metal hydride battery, which has a relatively high storage capacity per unit volume. An example of such a metal hydride battery is a battery comprising LaNi5 as the electrochemically active material, more specifically as a negative electrode. In the discharged condition, the negative electrode comprises LaNi5, while in the charged condition hydrogen is absorbed by said electrode, resulting in LaNi5H6. A disadvantage of this type of battery is the relatively low storage capacity per unit weight. The latter is caused by the high density of the metal hydride, which is about 7 g/cm3.
For some time now research has been directed to suitable metal hydrides with a lower density which can act as a hydrogen storage material. In theory, magnesium is very suitable for hydrogen absorption. However, the temperature at which the magnesium can be charged and discharged is 400xc2x0 C. Magnesium-nickel alloys appear to be better candidates for hydrogen storage material which can be used in electrochemical cells, as these alloys can be used for hydrogen absorption at lower temperatures.
The use of magnesium-nickel hydride for hydrogen storage is also disclosed in JP-56114801. According to the latter publication, said material can store hydrogen in a stable way at high temperatures. Although magnesium-nickel hydride can absorb a relatively large amount of hydrogen, it is as such not suitable as an electrochemically active material in an electrochemical cell. One of the reasons for this is the relatively slow kinetics of hydrogen absorption and hydrogen release.
In order to improve the kinetics of hydrogen absorption and hydrogen release to a sufficient level and to restrict corrosion, the amount of nickel added to a magnesium-nickel alloy has to be so high that only a small improvement is obtained in comparison with the known LaNi5 material.
The present invention aims to provide a hydrogen storage material which can be applied in an electrochemical cell and which comprises a magnesium-containing intermetallic compound capable of forming a hydride with hydrogen and having a high storage capacity per unit weight.
To this end, the present invention provides for a hydrogen storage material comprising a magnesium-containing intermetallic compound, which is characterized in that the intermetallic compound comprises an alloy of magnesium and a trivalent metal selected from the group of Sc, Y, La, and the rare earth elements.
It appears that alloys of magnesium with one of the above trivalent metals can be easily charged and discharged with hydrogen at room temperature. Moreover, the gravimetrical storage capacity of alloys of magnesium and a trivalent metal selected from the group of Sc, Y, La and the rare earth elements is considerably higher than the gravimetrical storage capacity of LaNi5. As a result, said alloys can advantageously be used as hydrogen storage materials in electrochemical cells. As rechargeable batteries are increasingly used in portable electronic equipment, the gravimetrical storage capacity is of the utmost importance.
In a particular embodiment, the intermetallic compound comprises an alloy selected from the group of scandium-magnesium, gadolinium-magnesium and yttrium-magnesium.
Among the above group of magnesium alloys, alloys of magnesium with the trivalent metals scandium, gadolinium and yttrium have relatively very low densities and accordingly relatively high gravimetrical storage capacities.
Preferably, the intermetallic compound comprises a scandium-magnesium alloy.
Scandium-magnesium alloys are capable of reversible absorption of hydrogen at room temperature. The amount of hydrogen which can be absorbed per unit weight is such that the gravimetrical storage capacity shows an improvement by more than a factor 4 in comparison with the gravimetrical storage capacity of the known LaNi5Hx.
RU-2072113 discloses a hydrogen-absorbing alloy of the AB5 type, comprising lanthanum and nickel, which additionally may contain a small amount of scandium. In fact, this material is completely different from the hydrogen storage material according to the present invention as the latter basically relates to a magnesium-scandium alloy without any lanthanum or nickel present.
Advantageously, the scandium-magnesium alloy comprises 1-50 at. % scandium and 50-99 at. % magnesium, more advantageously 15-40 at. % scandium and 60-85 at. % magnesium, and preferably 30-40 at. % scandium and 60-70 at. % magnesium.
The specific amounts of the different components in the alloy are determined by balancing the kinetics and the storage capacity against each other. As was mentioned above, magnesium has a high storage capacity. The kinetics of charging and discharging of the alloy can be improved by the addition of scandium.
Preferably, the scandium-magnesium alloy comprises Sc0.35Mg0.65Hx.
Said alloy provides for a very good balance between the hydrogen storage capacity and kinetics.
In a preferred embodiment, the hydrogen storage material according to the present invention comprises an amount of a catalytically active material.
Such a catalytically active material increases the kinetics of the hydrogen uptake of the hydrogen storage material.
Advantageously, the catalytically active material comprises at least one metal selected from the group consisting of palladium, platinum, cobalt, nickel, rhodium or iridium, and/or a composition of the formula DE3, wherein D is at least one element selected from the group consisting of Cr, Mo and W, and E is at least one element selected from the group consisting of Ni and Co.
Preferably, the catalytically active material comprises palladium, platinum or rhodium.
It has been found that the addition of, for example, only 0.6 at. % of palladium to the alloy increases the rate of hydrogen uptake by several orders of magnitude. The addition of 1.2 at. % palladium yields even better results in hydrogen uptake.
Furthermore, the present invention relates to an electrochemically active material which is characterized in that the material comprises a hydrogen storage material according to the present invention as described above.
The hydrogen storage material according to the present invention can advantageously be used in fuel cells.
The invention also relates to an electrochemical cell comprising a negative electrode. Said electrochemical cell is characterized in that the negative electrode comprises a hydrogen storage material as described above.
The electrochemical cell preferably comprises a rechargeable battery.
Finally, the present invention relates to electronic equipment powered by at least one electrochemical cell. Said electronic equipment is characterized in that the at least one electrochemical cell is an electrochemical cell as described above.